The present invention relates in general to high definition television (referred to hereinafter as HDTV) transmission systems, and more particularly to a cochannel interference filter for a HDTV transmission system.
In the case where a NTSC transmitter transmits a NTSC-coded signal at the same time that a HDTV transmitter transmits a HDTV-coded signal through a television channel of 6 MHZ, a HDTV receiver receives an undesirable strong interference component from the NTSC transmitter as well as the HDTV-coded signal from the HDTV transmitter, because of various factors including its physical position. The transmission of the desired HDTV signal with the undesirable signal through the same channel is usually called cochannel interference. Many HDTV transmission systems have been developed to remove cochannel interference, and one example is shown in FIG. 1.
Referring to FIG. 1, there is shown a block diagram of a conventional HDTV transmission system. As shown in this drawing, the conventional HDTV transmission system comprises a HDTV transmitter 100 and a HDTV receiver 200. The HDTV transmitter 100 comprises a video encoder 1 for band-compressing a video signal of 37 MHZ into a frequency band signal of 6 MHZ. The video signal contains red (R), green (G) and blue (B) color signals.
A data formatter/error control circuit 2 is provided in the HDTV transmitter 100 to receive the output signal from the video encoder 1, as well as an audio signal and auxiliary data, format the received signals and data in the NTSC format and perform a Reed-Solomon (R-S) coding for error correction of the received signals and data.
A pre-coder 3 is also provided in the HDTV transmitter 100 to code an output signal from the data formatter/error control circuit 2 in such a manner that 525 line data will be transmitted 60 times per second. The pre-coder 3 also acts to compensate for a characteristic of a post-comb filter 9, as will be described later in detail.
Also, the HDTV transmitter 100 comprises a modulator/SAW filter circuit 4 for VSB-modulating an output signal from the pre-coder 3, and a transmitter 5 for converting an output signal from the modulator/SAW filter circuit 4 into a radio frequency (RF) signal and transmitting the converted RF signal through a transmitting antenna.
The HDTV receiver 200 comprises a RF tuner/IF SAW filter circuit 6 for selecting a desired one of the RF signals received through a receiving antenna and converting the selected RF signal into an intermediate frequency (IF) signal with a band width of 6 MHZ, a demodulator 7 for demodulating the IF signal from the RF tuner/IF-SAW filter circuit 6 into a base band signal of 6 MHZ, and a data clock recovery circuit 8 for detecting a data clock and vertical and horizontal synchronous signals from the base band signal from the demodulator 7.
The post-comb filter 9 is provided in the HDTV receiver 200 to remove a NTSC cochannel interference component from an output signal from the data clock recovery circuit 8.
A channel equalizer 10 is also provided in the HDTV receiver 200 to compensate for a linear channel distortion such as a tilt, a ghost, an inter-symbol interference and etc.
An error correction circuit 11 is also provided in the HDTV receiver 200 to perform R-S decoding for error correction of an output signal from the channel equalizer 10.
Also, the HDTV receiver 200 comprises a deformatter 12 for separating a video signal, an audio signal and auxiliary data from an output signal from the error correction circuit 11, and a video decoder 13 for expanding the video signal of 6 MHz from the deformatter 12 into a frequency band signal of 37 MHZ.
FIGS. 2a to 2c are frequency spectra used to remove the NTSC cochannel interference component in accordance with the prior art. FIG. 2A is a frequency spectrum of a video carrier, a chrominance subcarrier and an audio carrier of a NTSC signal, FIG. 2b is a frequency spectrum illustrating the characteristic of the post-comb filter 9 in FIG. 1 and FIG. 2c is a frequency spectrum of a typical HDTV signal. In FIG. 2A, the video carrier V is 1.25 MHZ apart from one side of the channel, the chrominance subcarrier C is about 3.87 MHZ apart from the video carrier V and the audio carrier A is about 4.5 MHZ apart from the video carrier V.
A pilot signal of the HDTV signal is 571/2 f.sub.H apart from the video carrier and the NTSC carriers or the video carrier, the chrominance subcarrier and the audio carrier are present nearly in null points. In this connection, the NTSC carriers can be removed by the post-comb filter 9, so that a S/N ratio of the HDTV receiver can be enhanced. Here, f.sub.H is a NTSC horizontal line rate.
FIG. 3a is a block diagram of the pre-coder 3 in FIG. 1 and FIG. 3b is a block diagram of the post-comb filter 9 in FIG. 1 with the characteristic as shown in FIG. 2b. As shown in FIG. 3a, the pre-coder 3 includes a feedback circuit provided with a modulo-4 adder 17 and a delay circuit 18.. The modulo-4 adder 17 receives the output signal from the data formatter/error control circuit 2 at its first input and receives back its output signal delayed through the delay circuit 18 as its second input.
As shown in FIG. 3b, the post-comb filter 9 includes a feedforward circuit provided with a delay circuit 20 and an adder 22. The output signal from the data clock recovery circuit 8 is applied to the delay circuit 20. The output signal from the data clock recovery circuit 8 is also applied as a first input of the adder 22, which receives an output signal from the delay circuit 20 as its second input. The adder 22 acts to subtract the output signal from the delay circuit 20 from the output signal from the data clock recovery circuit 8. An output signal from the adder 22 is applied to a modulo-4 interpreter 23, which is provided between the post-comb filter 9 and the channel equalizer 10.
The delay circuits 18 and 20 have the same delay characteristic. Because the pre-coder 3 performs a pure digital operation, the delay characteristic of the delay circuit 18 must be an integer multiple of a data sampling rate fs, namely, D=N(1/fs).
In the case of DSC-HDTV, 2 level data or 4 level data may be used as the input to the feedback circuit of the pre-coder 3. If the 4 level data is passed through the feedback circuit, 7 level data can be obtained at the output. This results in an increase in the power of the HDTV transmitter. For this reason, the modulo-4 adder 17 is used instead of a general adder in the pre-coder 3. With the modulo-4 adder 17 used, the input 4 level data is directly outputted.
On the other hand, the post-comb filter 9 inputs 4 level data and outputs 7 level data. The 7 level data from the post-comb filter 9 is outputted as 4 level data by the modulo-4 interpreter 23.
However, the above-mentioned conventional HDTV transmission system has a disadvantage in that it cannot remove harmonic components of the video carrier, the chrominance subcarrier and the audio carrier of the NTSC signal since it is designed to remove only the video carrier, the chrominance subcarrier and the audio carrier of the NTSC signal. Also, the video carrier, the chrominance subcarrier and the audio carrier of the NTSC signal cannot completely be removed because they,are not completely positioned in the null points.